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I would recommend http://www.pingdom.com.
You can use pindom to monitor HTTP,
and various other services, you can
get it to send you alerts by email,
text, even DM you on Twitter.
You can also choose the level of the checks every 10 minutes 5 mintutes etc.
They will can also give you a report which you can link to on your website, which would give you an ...

we use Pingdom, there is a Free plan, it checks your site every 5 minutes, but you can check only one site, url : http://www.pingdom.com/
With BasicState you can check unlimited number of sites and check frequency is 15 minutes, url : http://basicstate.com/
Update:
There is another great site: http://www.monitor.us/free-IT-system-monitoring

I use UptimeRobot and have been very pleased. It's pretty basic monitoring (does an URL that you specify respond with 200OK and in a reasonable amount of time?), but it's free, and they offer alerts via email, SMS (via email gateways, which are sometimes delayed), and Twitter.
I recently had an outage while I happened to be using the server, and I was ...

Tracking offline conversion is tricky, but here are some potential strategies to measure the effect of an online ad on physical store sales:
Build a discount voucher system
Create a coupon code system to track the source of sales. This could be a simple offer code that visitors quote at the physical checkout, or a voucher to print. Here's a system to ...

We've been using this, Monitor Hacked Files, which is a PHP script that you install on the server. It notifies you if there are any changes to your site -- sending an email with a list of changed files. This at least tells you where to look. Definitely better than Google or clients finding out before you do!

Check out http://www.webpagetest.org/. You can run tests against your site from a remote server and it will tell you how long any page took to load and how long each component took to load.
The site uses multiple servers around the world so you will be able to check if you have speed differences due to location.
This is still in the VERY early stages ...

You can use Google Analytic's Campaign Tracking for that banner ad which will allow you to get specific information about users who click on that ad. It can't tell you who is actually buying anything since the sales occur offline but if they do eventually sell goods online that can be tracked as well.

Similar to TV commercials that say "Ask for agent 347" or "use promo code 219" you can ad something to your Advert that helps indicate which placement/location was the source for your lead.
If you combine this with Google Analytics (or similar) you can see where users are coming in from.
The only trick with this is that at large scale the accuracy drops. ...

We use a service like that from serviceuptime.com. Its been working well enough for us not to change. We do occasionally have issues with getting it to send out SMS alerts reliably and promptly.
We had something internal we hacked together before we used this outfit. One day the power went down and the generators at the hosting company failed and then the ...

Sucuri offers a monitoring system which will notify you via e-mail when a page is changed. It also has specific notices for malware infections that are being run through a website. They can also monitor DNS records, WHOIS information, and SSL certificates in relation to your domain and website. Their bare-bones basic service is free and their paid ...

You could always do it manually. Just save the current time in your boostrap and then log the difference at the end of page output. From there, you'll be able to plot live graphs to visualize the performance of pages you're interested in.
Or if you want an online service, here's a free one I just found called mon.itor.us. Offsite solutions like this will ...

You could set up a Google Alert to search your site for probable junk. For example, you might have the alert search for "viagra site:example.com". Google alerts will email you if it ever finds the target of your search.

mon.itor.us and SiteUptime have free/ad-supported plans. Check are only at 30-minute intervals however.
They both have plan upgrades which measure at 5 minute intervals and are cheaper than Pingdom, so you might find a reasonable cost/benefit trade-off.

One of our sites was recently hacked and we didn't notice for a while, because nothing was broken, until we became aware of our site appearing with spammy words in Google searches. Since our site was using git, I came up with a quick and dirty solution for monitoring the site for unauthorised file changes using the git diff command.
It is a bash script (but ...

A few simple options to automate malware scanning of multiple sites:
Use a paid automated malware scanning service such as Securi, 6Scan or SiteLock. You might want to encourage your client to sign up with these services, or offer it as part of a monthly maintenance contract.
Use a free plugin for your content management system, such as Wordfence for ...

I like the idea of using hash sums to check for changes in files. I don't like that the one you link to is installed on the server itself. If the server is hacked, that file is likely to be modified too.
I would use a command like this to ssh into each remote server and checksum all the important files in the web directory.
ssh mywebsite.example.com ...

Two tools that run on windows might work for you md5deep and AFICK - another file integrity checker. The allow the auditing and also become aware of new files.
AFAICK there is now web interface for them but you might want to send you the report via E-Mail right from the moment the tools flag a warning anyway.